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Physical Review A

Coherent population trapping in negatively charged self-assembled quantum dots using a train of femtosecond pulses

Parvendra Kumar, Takashi Nakajima

Published Feb 26, 2015• 6 citations
Publisher

Abstract

We demonstrate the coherent population trapping in a single quantum dot and an ensemble of negatively charged quantum dots using a train of femtosecond pulses. Particularly in an ensemble of quantum dots, we show that the detrimental effects due to the inhomogeneous distribution of their properties can be minimized by appropriately choosing the pulse-train parameters and the magnetic-field strength in such a way that the electron-Zeeman splitting is an integer multiple of the pulse repetition rate.

DOI
10.1103/physreva.91.023832

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We demonstrate the coherent population trapping in a single quantum dot and an ensemble of negatively charged quantum dots using a train of femtosecond pulses. Particularly in an ensemble of quantum dots, we show that the detrimental effects due to the inhomogeneous distribution of their properties can be minimized by appropriately choosing the pulse-train parameters and the magnetic-field strength in such a way that the electron-Zeeman splitting is an integer multiple of the pulse repetition rate.
Inhomogeneous broadening in ensembles of semiconductor quantum dots (QDs) has hindered coherent operations due to the detuning effects, caused by the large fluctuation in the QD transition energy due to size dispersion. This difficulty is especially evident when using femtosecond laser pulses for excitation. Here, the authors successfully measure detuning-dependent coherent evolution for a QD ensemble by employing prepulse two-dimensional coherent spectroscopy. The dephasing mechanism is found to be significantly different from that observed using picosecond lasers. This work presents a novel method to explore collective phenomena, such as superfluorescence, as well as motivating future studies of coherent interactions in the femtosecond regime.
In this Ph.D. Thesis we investigate the viability of using quantum dot ensembles as a quantum memory architecture through the use numerical simulations to study population transfer within quantum dots. This is followed by an investigation into the effects of high order wavemixing on the population transfer within two level systems, which was born from effects noted while simulating quantum dots. We study the initialisation of an ensemble of inhomogeneously broadened quantum dots, introducing a novel initialisation method utilising pump field with a slow frequency sweep. We focus on the properties of such an initialisation procedure and conclude that the maximum initialisation fidelities are determined entirely by the Zeeman splittings and decay rates of the quantum dots. We study several possibilities for performing π rotations on the population of an ensemble of quantum dots, and show the RCAP protocol is the most applicable. We study this protocol in the context of quantum dots and give the optimal parameters to use to generate high fidelity π pulses. We then bring together our work on quantum dots population transfer with the work of others covering the write and read procedures on quantum dots to provide a feasibility analysis of the complete quantum memory protocol. The work on wavemixing presented in this thesis uses a novel approach to analyse wavemixing effects which is used to predict the population transferred in two level simulations of wavemixing processes. We provide simulation confirmation of our approach to analyse wavemixing effects and then go on to calculate the disruptive effects of wavemixing caused by high intensity lasers on some simple systems. Finally we show that large orders of wavemixing can, at least in principle, be used for coherent population transfer.
Abstract The ability to extend the time scale of the coherent optical response from large ensembles of quantum emitters is highly appealing for applications in quantum information devices. In semiconductor nanostructures, spin degrees of freedom can be used as auxiliary, powerful tools to modify the coherent optical dynamics. Here, we apply this approach to negatively charged (In,Ga)As/GaAs self-assembled quantum dots which are considered as excellent quantum emitters with robust optical coherence and high bandwidth. We study three-pulse spin-dependent photon echoes subject to moderate transverse magnetic fields up to 1 T. We demonstrate that the timescale of coherent optical response can be extended by at least an order of magnitude by the field. Without magnetic field, the photon echo decays with T2 = 0.45 ns which is determined by the radiative lifetime of trions T1 = 0.26 ns. In the presence of the transverse magnetic field, the decay of the photon echo signal is given by spin dephasing time of the ensemble of resident electrons T2,e ∼ 4 ns. We demonstrate that the non-zero transverse g-factor of the heavy holes in the trion state plays a crucial role in the temporal evolution and magnetic field dependence of the long-lived photon echo signal.
The ability to extend the time scale of the coherent optical response from large ensembles of quantum emitters is highly appealing for applications in quantum information devices. In semiconductor nanostructures, spin degrees of freedom can be used as auxiliary, powerful tools to modify the coherent optical dynamics. Here, we apply this approach to negatively charged (In,Ga)As/GaAs self-assembled quantum dots which are considered as excellent quantum emitters with robust optical coherence and high bandwidth. We study 3-pulse spin-dependent photon echoes subject to moderate transverse magnetic fields up to 1 T. We demonstrate that the timescale of coherent optical response can be extended by at least an order of magnitude by the field. Without magnetic field, the photon echo decays with T2T_ 2T2​ = 0.45 ns which is determined by the radiative lifetime of trions T1T_1T1​ = 0.27 ns. In the presence of the transverse magnetic field, the decay of the photon echo signal is given by spin dephasing time of the ensemble of resident electrons T2,eT_{2,e}T2,e​ ~ 4 ns. We demonstrate that the non-zero transverse g-factor of the heavy holes in the trion state plays a crucial role in the temporal evolution and magnetic field dependence of the long-lived photon echo signal.
Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text B. Sun, X. Xu, P. R. Berman, D. G. Steel, A. S. Bracker, D. Gammon, and L. J. Sham, "Coherent Population Trapping of an Electron Spin in a Single Negatively Charged Quantum Dot," in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optica Publishing Group, 2009), paper IThB2. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article